apply ocean modifier to existing face/plane
OK I found out according to this answer: can I use ocean modifier on any custom shape? 
You need to subdivide your plane as much as possible otherwise it will have no effect. and then you set the Geometry to Displace. 
I used the knife tool to first make a rectangle and do some loopcuts and after this I manually took the knife tool to subdivide the parts that were near my "curly" edges.

My mistake was the subdivide (I tested the Displace but thought it didn't work because the waves were gone).

Hopefully useful for others!